Bicomponent Fibers and Methods for Making Them

ABSTRACT

The present invention relates to bicomponent polymer fibers, and to processes for forming those fibers. Bicomponent polymer fibers are described, having a core comprising a core polymer and a sheath comprising a sheath polymer, wherein the sheath polymer is a polyolefin having an Mw less than about 65,000 g/mol. The core polymer has an Mw at least about 20,000 g/mol greater than the Mw of the sheath polymer. Processes for forming bicomponent fibers are also described, comprising (i) forming a molten blend of a core polymer and a sheath polymer; (ii) extruding the molten polymer blend using an extrusion die having a length to diameter ratio greater than or equal to about 10 and under shear conditions sufficient to drive the sheath polymer to the die wall; and (iii) forming meltblown fibers having a core comprising the core polymer and a sheath comprising the sheath polymer.

BACKGROUND OF THE INVENTION

Propylene-based polymers and copolymers are well known in the art for their usefulness in the manufacture of meltblown nonwoven fibers. Such fibers have a wide variety of uses, particularly when formed into nonwoven fabrics and used in applications such as medical and hygiene products, clothing, filter media, and sorbent products, among others. Meltblown nonwoven fibers are particularly useful in hygiene products, such as baby diapers, adult incontinence products, and feminine hygiene products. One consideration with respect to these fibers, particularly in hygiene applications, is the ability to produce aesthetically pleasing fabrics having good leakage performance at a low cost. Good leakage performance is achieved via the elasticity of the elastic layers of the fabrics, which provides better fit and conformity to the wearer, resulting in fewer leaks. A common aesthetic issue with the production of propylene-based nonwoven fabrics is their often rubbery, tacky, or sticky feel. Further, tackiness or stickiness can lead to processing problems.

In the past, aesthetic issues resulting from rubbery or tacky feel of fibers and fabrics have been addressed in two ways. First, nonwoven fabrics formed from propylene-based fibers have been covered with aesthetically pleasing facing layers, resulting in multilayered compositions. The main drawback of such compositions is the increased cost that results from the need to purchase or manufacture facing layers, as well as from added complexity in the supply chain. Second, bicomponent fibers (sometimes referred to as “conjugate” fibers) have been prepared using a spinning process in which separate polymer streams are fed to a single die or spinneret in order to form fibers comprising two (or more) polymer compositions. The resulting fibers have properties of both polymer components. Bicomponent fibers are commonly classified by their cross-sectional structure. Such structures may include, but are not limited to, side-by-side, sheath-core, islands-in-the-sea or segmented-pie cross-sectional structures. Drawbacks of these traditionally produced bicomponent fibers are the high cost of the converting equipment required to allow two polymers to be independently fed to a single die, along with processing challenges (such as decreased throughput) which result from running two or more chemically and mechanically dissimilar polymers.

It would therefore be beneficial to produce fibers having desirable properties for use in elastic nonwoven fabrics while reducing cost and processing issues encountered in the past. The present invention involves blending a low molecular weight polymer component with a high molecular weight polymer component to form a polymer blend. Bicomponent fibers are then formed from the blends under shear conditions sufficient to cause the low molecular weight polymer to migrate to the outside of the fiber due to the molecular weight difference and limited miscibility of the polymers chosen for use in the blend. The resulting fibers have a core and sheath structure, with a core formed from the high molecular weight polymer and a sheath formed from the low molecular weight polymer.

SUMMARY OF THE INVENTION

The present invention relates to bicomponent polymer fibers, and to processes for forming those fibers. In some embodiments, the invention is directed to bicomponent polymer fibers having a core comprising a core polymer and a sheath comprising a sheath polymer, wherein the sheath polymer is a polyolefin having a weight average molecular weight (Mw) less than about 65,000 g/mol and the core polymer has an Mw at least about 20,000 g/mol greater than the Mw of the sheath polymer. In certain embodiments, the core polymer is a propylene-based polymer comprising from about 5 to about 30 wt % ethylene and/or a C₄-C₁₂ alpha olefin and having a triad tacticity greater than about 90% and a heat of fusion less than about 75 J/g, and the sheath polymer is an olefin wax. In other embodiments, the invention is directed to a process for forming bicomponent fibers comprising (i) forming a molten blend of a core polymer and a sheath polymer; (ii) extruding the molten polymer blend using an extrusion die having a length to diameter ratio greater than or equal to about 10 and under shear conditions sufficient to drive the sheath polymer to the die wall; and (iii) forming meltblown fibers having a core comprising the core polymer and a sheath comprising the sheath polymer. In such processes, the sheath polymer is a polyolefin having an Mw less than about 65,000 g/mol and the core polymer has an Mw at least about 20,000 g/mol greater than the Mw of the sheath polymer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to bicomponent polymer fibers having a core comprising a core polymer and a sheath comprising a sheath polymer, as well as, to processes for forming such fibers. The core polymer and the sheath polymer are selected so that the molecular weight difference in the two polymers, as well as their limited miscibility, allows the polymers to separate when subjected to shear forces during the extrusion process such that the core polymer remains in the center of the fiber while the sheath polymer migrates to the outside of the extrusion die and forms the outer covering of the fiber.

While the present application refers to “core” and “sheath” and “core polymers” and “sheath polymers”, these terms are used for convenience only. It should be noted that the polymers may not completely separate during the extrusion process, and there may not be a defined boundary between the core and sheath of the fibers described herein. Rather, fibers of the invention also include those fibers having a cross-sectional gradient, in which the concentration of the sheath polymer is highest at the surface of the fiber, and the concentration of the core polymer is highest at the center of the fiber.

As used herein, the term “copolymer” is meant to include polymers having two or more monomers, optionally with other monomers, and may refer to interpolymers, terpolymers, etc. The term “polymer” as used herein includes, but is not limited to, homopolymers, copolymers, terpolymers, etc. and alloys and blends thereof. The term “polymer” as used herein also includes impact, block, graft, random and alternating copolymers. The term “polymer” shall further include all possible geometrical configurations unless otherwise specifically stated. Such configurations may include isotactic, syndiotactic and random symmetries. The term “blend” as used herein refers to a mixture of two or more polymers.

The term “monomer” or “comonomer” as used herein can refer to the monomer used to form the polymer, i.e., the unreacted chemical compound in the form prior to polymerization, and can also refer to the monomer after it has been incorporated into the polymer, also referred to herein as a “[monomer]-derived unit”, which by virtue of the polymerization reaction typically has fewer hydrogen atoms than it does prior to the polymerization reaction. Different monomers are discussed herein, including propylene monomers, ethylene monomers, and diene monomers.

“Polypropylene” as used herein includes homopolymers and copolymers of propylene or mixtures thereof. Products that include one or more propylene monomers polymerized with one or more additional monomers may be more commonly known as random copolymers (RCP) or impact copolymers (ICP). Impact copolymers are also known in the art as heterophasic copolymers. “Propylene-based,” as used herein, is meant to include any polymer comprising propylene, either alone or in combination with one or more comonomers, in which propylene is the major component (e.g., greater than 50 wt % propylene). Likewise, “ethylene-based”, as used herein, is meant to include any polymer comprising ethylene, either alone or in combination with one or more comonomers, in which ethylene is the major component (e.g., greater than 50 wt % ethylene).

Core Polymer

Polymers intended for use as the core polymer in the fibers described herein include any polymers suitable for use in elastic nonwoven fabrics and articles. Such polymers typically include, but are not limited to, propylene-based polymers, ethylene-based polymers, styrenic block copolymers, propylene-ethylene block copolymers, acrylates, and combinations of the foregoing. In certain embodiments of the invention, the core polymer has a high molecular weight relative to the molecular weight of the sheath polymer. For example, in some embodiments the core polymer has a weight average molecular weight (M_(w)) that is at least about 20,000 g/mol greater, or at least about 50,000 g/mol greater, or at least about 75,000 g/mol greater, or at least about 100,000 g/mol greater than the M_(w) of the sheath polymer. In the same or other embodiments, the core polymer has an M_(w) greater than about 75,000 g/mol, or greater than about 100,000 g/mol, or greater than about 125,000 g/mol.

Propylene-Based Polymers

In certain embodiments of the present invention, the core polymer may comprise one or more propylene-based polymers, which comprise propylene and from about 5 to about 30 wt % of one or more comonomers selected from ethylene and/or C₄-C₁₂ α-olefins. In one or more embodiments, the α-olefin comonomer units may derive from ethylene, butene, pentene, hexene, 4-methyl-1-pentene, octene, or decene. The embodiments described below are discussed with reference to ethylene as the α-olefin comonomer, but the embodiments are equally applicable to other copolymers with other α-olefin comonomers. In this regard, the copolymers may simply be referred to as propylene-based polymers with reference to ethylene as the α-olefin.

In one or more embodiments, the propylene-based polymer may include at least about 5 wt %, or at least about 6 wt %, or at least about 7 wt %, or at least about 8 wt %, or at least about 10 wt %, or at least about 12 wt % ethylene-derived units. In those or other embodiments, the copolymers may include up to about 30 wt %, or up to about 25 wt %, or up to about 22 wt %, or up to about 20 wt %, or up to about 19 wt %, or up to about 18 wt %, or up to about 17 wt % ethylene-derived units, where the percentage by weight is based upon the total weight of the propylene-derived and α-olefin derived units. Stated another way, the propylene-based polymer may include at least about 70 wt %, or at least about 75 wt %, or at least about 80 wt %, or at least about 81 wt % propylene-derived units, or at least about 82 wt % propylene-derived units, or at least about 83 wt % propylene-derived units; and in these or other embodiments, the copolymers may include up to about 95 wt %, or up to about 94 wt %, or up to about 93 wt %, or up to about 92 wt %, or up to about 90 wt %, or up to about 88 wt % propylene-derived units, where the percentage by weight is based upon the total weight of the propylene-derived and alpha-olefin derived units. In certain embodiments, the propylene-based polymer may comprise from about 8 to about 20 wt % ethylene-derived units, or from about 12 to about 18 wt % ethylene-derived units.

The propylene-based polymers of one or more embodiments are characterized by a melting point (Tm), which can be determined by differential scanning calorimetry (DSC). For purposes herein, the maximum of the highest temperature peak is considered to be the melting point of the polymer. A “peak” in this context is defined as a change in the general slope of the DSC curve (heat flow versus temperature) from positive to negative, forming a maximum without a shift in the baseline where the DSC curve is plotted so that an endothermic reaction would be shown with a positive peak.

In one or more embodiments, the Tm of the propylene-based polymer (as determined by DSC) is less than about 115° C., or less than about 110° C., or less than about 100° C., or less than about 90° C.

In one or more embodiments, the propylene-based polymer may be characterized by its heat of fusion (Hf), as determined by DSC. In one or more embodiments, the propylene-based polymer may have an Hf that is at least about 0.5 J/g, or at least about 1.0 J/g, or at least about 1.5 J/g, or at least about 3.0 J/g, or at least about 4.0 J/g, or at least about 6.0 J/g, or at least about 7.0 J/g. In these or other embodiments, the propylene-based copolymer may be characterized by an Hf of less than about 75 J/g, or less than about 70 J/g, or less than about 60 J/g, or less than about 50 J/g, or less than about 30 J/g.

As used within this specification, DSC procedures for determining Tm and Hf include the following. The polymer is pressed at a temperature of from about 200° C. to about 230° C. in a heated press, and the resulting polymer sheet is hung, under ambient conditions, in the air to cool. About 6 to 10 mg of the polymer sheet is removed with a punch die. This 6 to 10 mg sample is annealed at room temperature for about 80 to 100 hours. At the end of this period, the sample is placed in a DSC (Perkin Elmer Pyris One Thermal Analysis System) and cooled to about −50° C. to about −70° C. The sample is heated at 10° C./min to attain a final temperature of about 200° C. The sample is kept at 200° C. for 5 minutes and a second cool-heat cycle is performed. Events from both cycles are recorded. The thermal output is recorded as the area under the melting peak of the sample, which typically occurs between about 0° C. and about 200° C. It is measured in Joules and is a measure of the Hf of the polymer.

The propylene-based polymer can have a triad tacticity of three propylene units, as measured by ¹³C NMR, of 75% or greater, 80% or greater, 85% or greater, 90% or greater, 92% or greater, 95% or greater, or 97% or greater. In one or more embodiments, the triad tacticity may range from about 75 to about 99%, or from about 80 to about 99%, or from about 85 to about 99%, or from about 90 to about 99%, or from about 90 to about 97%, or from about 80 to about 97%. Triad tacticity is determined by the methods described in U.S. Patent Application Publication No. 2004/0236042.

The propylene-based polymer may have a tacticity index m/r ranging from a lower limit of 4 or 6 to an upper limit of 8 or 10 or 12. The tacticity index, expressed herein as “m/r”, is determined by ¹³C nuclear magnetic resonance (“NMR”). The tacticity index m/r is calculated as defined by H. N. Cheng in 17 MACROMOLECULES, pp. 1950-1955 (1984), incorporated herein by reference. The designation “m” or “r” describes the stereochemistry of pairs of contiguous propylene groups, “m” referring to meso and “r” to racemic. An m/r ratio of 1.0 generally describes a syndiotactic polymer, and an m/r ratio of 2.0 generally describes an atactic material. An isotactic material theoretically may have a ratio approaching infinity, and many by-product atactic polymers have sufficient isotactic content to result in ratios of greater than 50.

In one or more embodiments, the propylene-based polymer may have a crystallinity of from about 0.5% to about 40%, or from about 1% to about 30%, or from about 5% to about 25%, determined according to DSC procedures. Crystallinity may be determined by dividing the Hf of a sample by the Hf of a 100% crystalline polymer, which is assumed to be 189 joules/gram for isotactic polypropylene or 350 joules/gram for polyethylene.

In one or more embodiments, the propylene-based polymer may have a density of from about 0.85 g/cm³ to about 0.92 g/cm³, or from about 0.86 g/cm³ to about 0.90 g/cm³, or from about 0.86 g/cm³ to about 0.89 g/cm³ at room temperature as measured per the ASTM D-792 test method.

In one or more embodiments, the propylene-based polymer can have a melt index (MI) (ASTM D-1238, 2.16 kg @ 190° C.), of less than or equal to about 100 g/10 min, or less than or equal to about 50 g/10 min, or less than or equal to about 25 g/10 min, or less than or equal to about 10 g/10 min, or less than or equal to about 9.0 g/10 min, or less than or equal to about 8.0 g/10 min, or less than or equal to about 7.0 g/10 min.

In one or more embodiments, the propylene-based polymer may have a melt flow rate (MFR), as measured according to ASTM D-1238, 2.16 kg weight @ 230° C., greater than about 1 g/10 min, or greater than about 2 g/10 min, or greater than about 5 g/10 min, or greater than about 8 g/10 min, or greater than about 10 g/10 min. In the same or other embodiments, the propylene-based polymer may have an MFR less than about 500 g/10 min, or less than about 400 g/10 min, or less than about 300 g/10 min, or less than about 200 g/10 min, or less than about 100 g/10 min, or less than about 50 g/10 min, or less than about 25 g/10 min. In certain embodiments, the propylene-based polymer may have an MFR from about 1 to about 100 g/10 min, or from about 2 to about 50 g/10 min, or from about 5 to about 25 g/10 min.

In one or more embodiments, the propylene-based polymer may have a Mooney viscosity [ML (1+4) @ 125° C.], as determined according to ASTM D-1646, of less than about 100, or less than about 75, or less than about 50, or less than about 30.

In one or more embodiments, the first and second polymers may have a g′ index value of 0.95 or greater, or at least 0.97, or at least 0.99, wherein g′ is measured at the Mw of the polymer using the intrinsic viscosity of isotactic polypropylene as the baseline. For use herein, the g′ index is defined as:

-   -   where η_(b) is the intrinsic viscosity of the polymer and η_(l)         is the intrinsic viscosity of a linear polymer of the same         viscosity-averaged molecular weight (M_(v)) as the polymer.         η_(l)=KW_(v) ^(α), K and α are measured values for linear         polymers and should be obtained on the same instrument as the         one used for the g′ index measurement.

In one or more embodiments, the propylene-based copolymer can have a weight average molecular weight (Mw) of from about 50,000 to about 5,000,000 g/mol, or from about 75,000 to about 1,000,000 g/mol, or from about 100,000 to about 500,000 g/mol, or from about 125,000 to about 300,000 g/mol.

In one or more embodiments, the propylene-based copolymer can have a number average molecular weight (Mn) of from about 2,500 to about 2,500,000 g/mole, or from about 5,000 to about 500,000 g/mole, or from about 10,000 to about 250,000 g/mole, or from about 25,000 to about 200,000 g/mole.

In one or more embodiments, the propylene-based copolymer can have a Z-average molecular weight (Mz) of from about 10,000 to about 7,000,000 g/mole, or from about 50,000 to about 1,000,000 g/mole, or from about 80,000 to about 700,000 g/mole, or from about 100,000 to about 500,000 g/mole.

In one or more embodiments, the molecular weight distribution (MWD, equal to Mw/Mn) of the propylene-based copolymer may be from about 1 to about 40, or from about 1 to about 15, or from about 1.8 to about 5, or from about 1.8 to about 3.

Techniques for determining the molecular weight (Mn, Mw and Mz) and MWD may be found in U.S. Pat. No. 4,540,753 (Cozewith, Ju and Verstrate) (which is incorporated by reference herein for purposes of U.S. practices) and references cited therein and in Macromolecules, 1988, Vol. 21, pp. 3360-3371 (Ver Strate et al.), which is herein incorporated by reference for purposes of U.S. practices, and references cited therein. For example, molecular weight may be determined by size exclusion chromatography (SEC) by using a Waters 150 gel permeation chromatograph equipped with the differential refractive index detector and calibrated using polystyrene standards.

Optionally, the propylene-based polymer may also include one or more dienes. The term “diene” is defined as a hydrocarbon compound that has two unsaturation sites, i.e., a compound having two double bonds connecting carbon atoms. Depending on the context, the term “diene” in this patent refers broadly to either a diene monomer prior to polymerization, e.g., forming part of the polymerization medium, or a diene monomer after polymerization has begun (also referred to as a diene monomer unit or a diene-derived unit). Exemplary dienes suitable for use in the present invention include, but are not limited to, butadiene, pentadiene, hexadiene (e.g., 1,4-hexadiene), heptadiene (e.g., 1,6-heptadiene), octadiene (e.g., 1,7-octadiene), nonadiene (e.g., 1,8-nonadiene), decadiene (e.g., 1,9-decadiene), undecadiene (e.g., 1,10-undecadiene), dodecadiene (e.g., 1,11-dodecadiene), tridecadiene (e.g., 1,12-tridecadiene), tetradecadiene (e.g., 1,13-tetradecadiene), pentadecadiene, hexadecadiene, heptadecadiene, octadecadiene, nonadecadiene, icosadiene, heneicosadiene, docosadiene, tricosadiene, tetracosadiene, pentacosadiene, hexacosadiene, heptacosadiene, octacosadiene, nonacosadiene, triacontadiene, and polybutadienes having a molecular weight (Mw) of less than 1,000 g/mol. Examples of straight chain acyclic dienes include, but are not limited to 1,4-hexadiene and 1,6-octadiene. Examples of branched chain acyclic dienes include, but are not limited to 5-methyl-1,4-hexadiene, 3,7-dimethyl-1,6-octadiene, and 3,7-dimethyl-1,7-octadiene. Examples of single ring alicyclic dienes include, but are not limited to 1,4-cyclohexadiene, 1,5-cyclooctadiene, and 1,7-cyclododecadiene. Examples of multi-ring alicyclic fused and bridged ring dienes include, but are not limited to tetrahydroindene; norbornadiene; methyltetrahydroindene; dicyclopentadiene; bicyclo(2.2.1)hepta-2,5-diene; and alkenyl-, alkylidene-, cycloalkenyl-, and cycloalkylidene norbornenes [including, e.g., 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 5-propenyl-2-norbornene, 5-isopropylidene-2-norbornene, 5-(4-cyclopentenyl)-2-norbornene, 5-cyclohexylidene-2-norbornene, and 5-vinyl-2-norbornene]. Examples of cycloalkenyl-substituted alkenes include, but are not limited to vinyl cyclohexene, allyl cyclohexene, vinylcyclooctene, 4-vinylcyclohexene, allyl cyclodecene, vinylcyclododecene, and tetracyclododecadiene. In some embodiments of the present invention, the diene is selected from 5-ethylidene-2-norbornene (ENB); 1,4-hexadiene; 5-methylene-2-norbornene (MNB); 1,6-octadiene; 5-methyl-1,4-hexadiene; 3,7-dimethyl-1,6-octadiene; 1,3-cyclopentadiene; 1,4-cyclohexadiene; vinyl norbornene (VNB); dicyclopentadiene (DCPD), and combinations thereof. In one or more embodiments, the diene is ENB.

In some embodiments, the propylene-based polymer may comprise from 0.05 to about 6 wt % diene-derived units. In further embodiments, the polymer comprises from about 0.1 to about 5.0 wt % diene-derived units, or from about 0.25 to about 3.0 wt % diene-derived units, or from about 0.5 to about 1.5 wt % diene-derived units.

In one or more embodiments, the propylene-based polymer can be grafted (e.g., “functionalized”) using one or more grafting monomers. As used herein, the term “grafting” denotes covalent bonding of the grafting monomer to a polymer chain of the propylene-based polymer.

The grafting monomer can be or include at least one ethylenically unsaturated carboxylic acid or acid derivative, such as an acid anhydride, ester, salt, amide, imide, acrylates or the like. Illustrative monomers include but are not limited to acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, maleic anhydride, 4-methyl cyclohexene-1,2-dicarboxylic acid anhydride, bicyclo(2.2.2)octene-2,3-dicarboxylic acid anhydride, 1,2,3,4,5,8,9,10-octahydronaphthalene-2,3-dicarboxylic acid anhydride, 2-oxa-1,3-diketospiro(4.4)nonene, bicyclo(2.2.1)heptene-2,3-dicarboxylic acid anhydride, maleopimaric acid, tetrahydrophthalic anhydride, norbornene-2,3-dicarboxylic acid anhydride, nadic anhydride, methyl nadic anhydride, himic anhydride, methyl himic anhydride, and 5-methylbicyclo(2.2.1)heptene-2,3-dicarboxylic acid anhydride. Other suitable grafting monomers include methyl acrylate and higher alkyl acrylates, methyl methacrylate and higher alkyl methacrylates, acrylic acid, methacrylic acid, hydroxy-methyl methacrylate, hydroxyl-ethyl methacrylate and higher hydroxy-alkyl methacrylates and glycidyl methacrylate. Maleic anhydride is a preferred grafting monomer.

In one or more embodiments, the grafted propylene based polymer comprises from about 0.5 to about 10 wt % ethylenically unsaturated carboxylic acid or acid derivative, more preferably from about 0.5 to about 6 wt %, more preferably from about 0.5 to about 3 wt %; in other embodiments from about 1 to about 6 wt %, more preferably from about 1 to about 3 wt %. In a preferred embodiment wherein the graft monomer is maleic anhydride, the maleic anhydride concentration in the grafted polymer is preferably in the range of about 1 to about 6 wt %, preferably at least about 0.5 wt % and highly preferably about 1.5 wt %.

Preparation of Propylene-Based Polymers

Polymerization of the propylene-based polymer is conducted by reacting monomers in the presence of a catalyst system described herein at a temperature of from 0° C. to 200° C. for a time of from 1 second to 10 hours. Preferably homogeneous conditions are used, such as a continuous solution process or a bulk polymerization process with excess monomer used as diluent. The continuous process may use some form of agitation to reduce concentration differences in the reactor and maintain steady state polymerization conditions. The heat of the polymerization reaction is preferably removed by cooling of the polymerization feed and allowing the polymerization to heat up to the polymerization, although internal cooling systems may be used.

Further description of exemplary methods suitable for preparation of the propylene-based polymers described herein may be found in U.S. Pat. No. 6,881,800, which is incorporated by reference herein for purposes of U.S. practice.

The triad tacticity and tacticity index of the propylene-based copolymer may be controlled by the catalyst, which influences the stereoregularity of propylene placement, the polymerization temperature, according to which stereoregularity can be reduced by increasing the temperature, and by the type and amount of a comonomer, which tends to reduce the level of longer propylene derived sequences.

Too much comonomer may reduce the crystallinity provided by the crystallization of stereoregular propylene derived sequences to the point where the material lacks strength; too little and the material may be too crystalline. The comonomer content and sequence distribution of the polymers can be measured using ¹³C nuclear magnetic resonance (NMR) by methods well known to those skilled in the art. Comonomer content of discrete molecular weight ranges can be measured using methods well known to those skilled in the art, including Fourier Transform Infrared Spectroscopy (FTIR) in conjunction with samples by GPC, as described in Wheeler and Willis, Applied Spectroscopy, 1993, Vol. 47, pp. 1128-1130. For a propylene ethylene copolymer containing greater than 75 wt % propylene, the comonomer content (ethylene content) of such a polymer can be measured as follows: A thin homogeneous film is pressed at a temperature of about 150° C. or greater, and mounted on a Perkin Elmer PE 1760 infrared spectrophotometer. A full spectrum of the sample from 600 cm-1 to 4000 cm-1 is recorded and the monomer weight percent of ethylene can be calculated according to the following equation: Ethylene wt %=82.585−111.987X+30.045X2, where X is the ratio of the peak height at 1155 cm-1 and peak height at either 722 cm-1 or 732 cm-1, whichever is higher. For propylene ethylene copolymers having 75 wt % or less propylene content, the comonomer (ethylene) content can be measured using the procedure described in Wheeler and Willis.

Reference is made to U.S. Pat. No. 6,525,157, whose test methods are also fully applicable for the various measurements referred to in this specification and claims and which contain more details on GPC measurements, the determination of ethylene content by NMR and the DSC measurements.

The catalyst may also control the stereoregularity in combination with the comonomer and the polymerization temperature. The propylene-based polymers described herein are prepared using one or more catalyst systems. As used herein, a “catalyst system” comprises at least a transition metal compound, also referred to as catalyst precursor, and an activator. Contacting the transition metal compound (catalyst precursor) and the activator in solution upstream of the polymerization reactor or in the polymerization reactor of the disclosed processes yields the catalytically active component (catalyst) of the catalyst system. Any given transition metal compound or catalyst precursor can yield a catalytically active component (catalyst) with various activators, affording a wide array of catalysts deployable in the processes of the present invention. Catalyst systems of the present invention comprise at least one transition metal compound and at least one activator. However, catalyst systems of the current disclosure may also comprise more than one transition metal compound in combination with one or more activators. Such catalyst systems may optionally include impurity scavengers. Each of these components is described in further detail below.

In one or more embodiments of the present invention, the catalyst systems used for producing propylene-based polymers comprise a metallocene compound. In some embodiments, the metallocene compound is a bridged bisindenyl metallocene having the general formula (In¹)Y(In²)MX₂, where In¹ and In² are identical substituted or unsubstituted indenyl groups bound to M and bridged by Y, Y is a bridging group in which the number of atoms in the direct chain connecting In¹ with In² is from 1 to 8 and the direct chain comprises C or Si, and M is a Group 3, 4, 5, or 6 transition metal. In¹ and In² may be substituted or unsubstituted. If In¹ and In² are substituted by one or more substituents, the substituents are selected from the group consisting of a halogen atom, C₁ to C₁₀ alkyl, C₅ to C₁₅ aryl, C₆ to C₂₅ alkylaryl, and N- or P-containing alkyl or aryl. Exemplary metallocene compounds of this type include, but are not limited to, μ-dimethylsilylbis(indenyl)hafniumdimethyl and μ-dimethylsilylbis(indenyl)zirconiumdimethyl.

In other embodiments, the metallocene compound may be a bridged bisindenyl metallocene having the general formula (In¹)Y(In²)MX₂, where In¹ and In² are identical 2,4-substituted indenyl groups bound to M and bridged by Y, Y is a bridging group in which the number of atoms in the direct chain connecting In¹ with In² is from 1 to 8 and the direct chain comprises C or Si, and M is a Group 3, 4, 5, or 6 transition metal. In¹ and In² are substituted in the 2 position by a methyl group and in the 4 position by a substituent selected from the group consisting of C₅ to C₁₅ aryl, C₆ to C₂₅ alkylaryl, and N- or P-containing alkyl or aryl. Exemplary metallocene compounds of this type include, but are not limited to, (μ-dimethylsilyl)bis(2-methyl-4-(3,′5′-di-tert-butylphenyl)indenyl)zirconiumdimethyl, (μ-dimethylsilyl)bis(2-methyl-4-(3,′5′-di-tert-butylphenyl)indenyl)hafniumdimethyl, (μ-dimethylsilyl)bis(2-methyl-4-naphthylindenyl)zirconiumdimethyl, (μ-dimethylsilyl)bis(2-methyl-4-naphthylindenyl)hafniumdimethyl, (μ-dimethylsilyl)bis(2-methyl-4-(N-carbazyl)indenyl)zirconiumdimethyl, and (μ-dimethylsilyl)bis(2-methyl-4-(N-carbazyl)indenyl)hafniumdimethyl.

Alternatively, in one or more embodiments of the present invention, the metallocene compound may correspond to one or more of the formulas disclosed in U.S. Pat. No. 7,601,666. Such metallocene compounds include, but are not limited to, dimethylsilyl bis(2-(methyl)-5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenz(f)indenyl)hafnium dimethyl, diphenylsilyl bis(2-(methyl)-5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenz(f)indenyl)hafnium dimethyl, diphenylsilyl bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenz(f)indenyl)hafnium dimethyl, diphenylsilyl bis(2-(methyl)-5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenz(f)indenyl)zirconium dichloride, and cyclo-propylsilyl bis(2-(methyl)-5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenz(f)indenyl)hafnium dimethyl.

In one or more embodiments of the present invention, the activators of the catalyst systems used to produce propylene-based polymers comprise a cationic component. In some embodiments, the cationic component has the formula [R¹R²R³AH]⁺, where A is nitrogen, R¹ and R² are together a —(CH₂)_(a)— group, where a is 3, 4, 5 or 6 and form, together with the nitrogen atom, a 4-, 5-, 6- or 7-membered non-aromatic ring to which, via adjacent ring carbon atoms, optionally one or more aromatic or heteroaromatic rings may be fused, and R³ is C₁, C₂, C₃, C₄ or C₅ alkyl, or N-methylpyrrolidinium or N-methylpiperidinium. In other embodiments, the cationic component has the formula [R_(n)AH]⁺, where A is nitrogen, n is 2 or 3, and all R are identical and are C₁ to C₃ alkyl groups, such as for example trimethylammonium, trimethylanilinium, triethylammonium, dimethylanilinium, or dimethylammonium.

In one or more embodiments of the present invention, the activators of the catalyst systems used to produce the propylene-based polymers comprise an anionic component, [Y]⁻. In some embodiments, the anionic component is a non-coordinating anion (NCA), having the formula [B(R⁴)₄]⁻, where R⁴ is an aryl group or a substituted aryl group, of which the one or more substituents are identical or different and are selected from the group consisting of alkyl, aryl, a halogen atom, halogenated aryl, and haloalkylaryl groups. In one or more embodiments, the substituents are perhalogenated aryl groups, or perfluorinated aryl groups, including but not limited to perfluorophenyl, perfluoronaphthyl and perfluorobiphenyl.

Together, the cationic and anionic components of the catalysts systems described herein form an activator compound. In one or more embodiments of the present invention, the activator may be N,N-dimethylanilinium-tetra(perfluorophenyl)borate, N,N-dimethylanilinium-tetra(perfluoronaphthyl)borate, N,N-dimethylanilinium-tetrakis(perfluorobiphenyl)borate, N,N-dimethylanilinium-tetrakis(3,5-bis(trifluoromethyl)phenyl)borate, triphenylcarbenium-tetra(perfluorophenyl)borate, triphenylcarbenium-tetra(perfluoronaphthyl)borate, triphenylcarbenium-tetrakis(perfluorobiphenyl)borate, or triphenylcarbenium-tetrakis(3,5-bis(trifluoromethyl)phenyl)borate.

Any catalyst system resulting from any combination of a metallocene compound, a cationic activator component, and an anionic activator component mentioned in the preceding paragraphs shall be considered to be explicitly disclosed herein and may be used in accordance with the present invention in the polymerization of one or more olefin monomers. Also, combinations of two different activators can be used with the same or different metallocene(s).

Suitable activators for the processes of the present invention also include alominoxanes (or alumoxanes) and aluminum alkyls. Without being bound by theory, an alumoxane is typically believed to be an oligomeric aluminum compound represented by the general formula (R^(x)—Al—O)_(n), which is a cyclic compound, or R^(x) (R^(x)—Al—O)_(n)AlR^(x) ₂, which is a linear compound. Most commonly, alumoxane is believed to be a mixture of the cyclic and linear compounds. In the general alumoxane formula, R^(x) is independently a C₁-C₂₀ alkyl radical, for example, methyl, ethyl, propyl, butyl, pentyl, isomers thereof, and the like, and n is an integer from 1-50. In one or more embodiments, R^(x) is methyl and n is at least 4. Methyl alumoxane (MAO), as well as modified MAO containing some higher alkyl groups to improve solubility, ethyl alumoxane, iso-butyl alumoxane, and the like are useful for the processes disclosed herein.

Further, the catalyst systems suitable for use in the present invention may contain, in addition to the transition metal compound and the activator described above, additional activators (co-activators) and/or scavengers. A co-activator is a compound capable of reacting with the transition metal complex, such that when used in combination with an activator, an active catalyst is formed. Co-activators include alumoxanes and aluminum alkyls.

In some embodiments of the invention, scavengers may be used to “clean” the reaction of any poisons that would otherwise react with the catalyst and deactivate it. Typical aluminum or boron alkyl components useful as scavengers are represented by the general formula R^(x)JZ₂ where J is aluminum or boron, R^(x) is a C₁-C₂₀ alkyl radical, for example, methyl, ethyl, propyl, butyl, pentyl, and isomers thereof, and each Z is independently R^(x) or a different univalent anionic ligand such as halogen (Cl, Br, I), alkoxide (OR^(x)) and the like. Exemplary aluminum alkyls include triethylaluminum, diethylaluminum chloride, ethylaluminium dichloride, tri-iso-butylaluminum, tri-n-octylaluminum, tri-n-hexylaluminum, trimethylaluminum and combinations thereof. Exemplary boron alkyls include triethylboron. Scavenging compounds may also be alumoxanes and modified alumoxanes including methylalumoxane and modified methylalumoxane.

In some embodiments, the catalyst system used to produce the propylene-based polymers comprises a transition metal component which is a bridged bisindenyl metallocene having the general formula (In¹)Y(In²)MX₂, where In¹ and In² are identical substituted or unsubstituted indenyl groups bound to M and bridged by Y, Y is a bridging group in which the number of atoms in the direct chain connecting In¹ with In² is from 1 to 8 and the direct chain comprises C or Si, and M is a Group 3, 4, 5, or 6 transition metal. In¹ and In² may be substituted or unsubstituted. If In¹ and In² are substituted by one or more substituents, the substituents are selected from the group consisting of a halogen atom, C₁ to C₁₀ alkyl, C₅ to C₁₅ aryl, C₆ to C₂₅ alkylaryl, and N- or P-containing alkyl or aryl. In one or more embodiments, the transition metal component used to produce the propylene-based polymers is μ-dimethylsilylbis(indenyl)hafniumdimethyl.

Ethylene-Based Polymers

In further embodiments of the present invention, the core polymer may comprise one or more ethylene-based polymers, which may be ethylene homopolymers and/or ethylene copolymers incorporating one or more comonomers. Various types of ethylene-based polymers are known in the art. Exemplary ethylene-based polymers include ethylene-propylene copolymers, low-density polyethylene (“LDPE”), linear low-density polyethylene (“LLDPE”), and high-density polyethylene (“HDPE”).

In at least one specific embodiment, the core polymer may be or include one or more ethylene-propylene copolymers (EP). Preferably, the EP is non-crystalline, e.g., atactic or amorphous, but in certain embodiments the EP may be crystalline (including “semi-crystalline”). The crystallinity of the EP is preferably derived from the ethylene, and a number of published methods, procedures and techniques are available for evaluating whether the crystallinity of a particular material is derived from ethylene. The crystallinity of the EP can be distinguished from the crystallinity of the propylene-based polymer by removing the EP from the composition and then measuring the crystallinity of the residual propylene-based polymer. Such crystallinity measured is usually calibrated using the crystallinity of polyethylene and related to the comonomer content. The percent crystallinity in such cases is measured as a percentage of polyethylene crystallinity and thus the origin of the crystallinity from ethylene is established.

In one or more embodiments, the EP can include one or more optional polyenes, including particularly a diene; thus, the EP can be an ethylene-propylene-diene (commonly called “EPDM”). The optional polyene is considered to be any hydrocarbon structure having at least two unsaturated bonds wherein at least one of the unsaturated bonds is readily incorporated into a polymer. The second bond may partially take part in polymerization to form long chain branches but preferably provides at least some unsaturated bonds suitable for subsequent curing or vulcanization in post polymerization processes. Examples of EP or EPDM copolymers include V722, V3708P, MDV 91-9, V878 that are available under the trade name Vistalon™ EPDM from ExxonMobil Chemical Company. Additionally, several commercial EPDM polymers are available from The Dow Chemical Co. under the trade names Nordel IP and MG.

Examples of the optional polyene include, but are not limited to, butadiene, pentadiene, hexadiene (e.g., 1,4-hexadiene), heptadiene (e.g., 1,6-heptadiene), octadiene (e.g., 1,7-octadiene), nonadiene (e.g., 1,8-nonadiene), decadiene (e.g., 1,9-decadiene), undecadiene (e.g., 1,10-undecadiene), dodecadiene (e.g., 1,11-dodecadiene), tridecadiene (e.g., 1,12-tridecadiene), tetradecadiene (e.g., 1,13-tetradecadiene), pentadecadiene, hexadecadiene, heptadecadiene, octadecadiene, nonadecadiene, icosadiene, heneicosadiene, docosadiene, tricosadiene, tetracosadiene, pentacosadiene, hexacosadiene, heptacosadiene, octacosadiene, nonacosadiene, triacontadiene, and polybutadienes having a molecular weight (Mw) of less than 1000 g/mol. Examples of straight chain acyclic dienes include, but are not limited to 1,4-hexadiene and 1,6-octadiene. Examples of branched chain acyclic dienes include, but are not limited to 5-methyl-1,4-hexadiene, 3,7-dimethyl-1,6-octadiene, and 3,7-dimethyl-1,7-octadiene. Examples of single ring alicyclic dienes include, but are not limited to 1,4-cyclohexadiene, 1,5-cyclooctadiene, and 1,7-cyclododecadiene. Examples of multi-ring alicyclic fused and bridged ring dienes include, but are not limited to, tetrahydroindene; norbornadiene; methyltetrahydroindene; dicyclopentadiene; bicyclo(2.2.1)hepta-2,5-diene; and alkenyl-, alkylidene-, cycloalkenyl-, and cycloalkylidene norbornenes [including, e.g., 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 5-propenyl-2-norbornene, 5-isopropylidene-2-norbornene, 5-(4-cyclopentenyl)-2-norbornene, 5-cyclohexylidene-2-norbornene, and 5-vinyl-2-norbornene]. Examples of cycloalkenyl-substituted alkenes include, but are not limited to vinyl cyclohexene, allyl cyclohexene, vinylcyclooctene, 4-vinylcyclohexene, allyl cyclodecene, vinylcyclododecene, and tetracyclododecadiene.

LLDPE is typically a copolymer of ethylene and one or more other alpha-olefins. Such alpha-olefins will generally have 3 to 20 carbon atoms. In certain embodiments, the alpha-olefins are selected from butene-1, pentene-1,4-methyl-1-pentene, hexene-1, octene-1, decene-1, and combinations thereof. In other embodiments, the alpha-olefins are selected from butene-1, hexene-1, octene-1, and combinations thereof. LLDPEs intended for use herein may be produced from any suitable catalyst system including conventional Ziegler-Natta type catalyst systems and metallocene based catalyst systems. In certain embodiments, LLDPE polymers may have a density from about 0.89 g/cm³ to 0.94 g/cm³, or from about 0.91 g/cm³ to about 0.94 g/cm³. Exemplary metallocene catalyzed linear low density polyethylenes include those available commercially from ExxonMobil Chemical Company under the name Exceed™ mPE resin.

HDPE is a semicrystalline polymer available in a wide range of molecular weights as indicated by either MI or HLMI (melt index or high-load melt index) and typically has an ethylene content of at least 99 mole percent (based upon the total moles of HDPE). If incorporated into the HDPE, comonomers may be selected from butene and other C₃ to C₂₀ alpha olefins. In one embodiment, the comonomers are selected from 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene, and mixtures thereof. In certain embodiments, comonomers may be present in the HDPE up to about 0.68 mole percent, based on the total moles of the HDPE. In further embodiments, comonomers are present in the HDPE up to about 0.28 mole percent. The density of HDPE is typically greater than 0.94 g/cm³. In some embodiments, the HDPE may have a density from about 0.94 g/cm³ to about 0.97 g/cm³, or from about 0.95 g/cm³ to about 0.965 g/cm³. In the same or other embodiments, the melting point of the HDPE, as measured by a differential scanning calorimeter (DSC), may be from about 120° C. to about 150° C., or from about 125° C. to about 135° C. Further, the HDPE may have a melt index from about 0.1 g/10 min to about 10.0 g/10 min, or from about 0.2 g/10 min to about 5.0 g/10 min, or from about 0.6 g/10 min to about 2.0 g/10 min.

HDPE includes polymers made using a variety of catalyst systems including Ziegler-Natta, Phillips-type catalysts, chromium based catalysts, and metallocene catalyst systems, which may be used with alumoxane and/or ionic activators. Processes useful for preparing such polyethylenes include gas phase, slurry, solution processes, and the like. Exemplary HDPEs include, but are not limited to, those commercially available as Marlex TR-130 from Phillips Chemical Company, M6211 from Equistar Chemical Co., Dow XU 6151.302 from Dow Chemical Co., and HD 7845, HD 6733, HTA 002, HTA 108, HYA 108, Paxon 4700, AD60 007, AA 45004, BA50 100, Nexxstar™ 0111 and MA001 from ExxonMobil Chemical Company.

Styrenic Block Copolymers

In further embodiments of the present invention, the core polymer may comprise one or more styrenic block copolymers (SBCs). The phrase “block copolymer” is intended to include any manner of block copolymer, including but not limited to diblock, triblock, and tetrablock copolymers. “Block copolymer” is further meant to include copolymers having any structure known to those of skill in the art, including but not limited to linear, radial or multi-arm star, or multi-branched block copolymers.

Suitable SBCs include block copolymers of styrene and one or more conjugated dienes such as SI (styrene-isoprene), SIS (styrene-isoprene-styrene), SB (styrene-butadiene), SBS (styrene-butadiene-styrene), and styrene-isoprene-butadiene (SIB). Styrene block copolymers comprising tetrablock or pentablock copolymers selected from SISI, SISB, SBSB, SBSI, ISISI, ISISB, BSISB, ISBSI, BSBSB, and BSBSI are also suitable. The block copolymers may or may not be hydrogenated.

In one or more embodiments, the SBC may comprise a styrene-isoprene-styrene (SIS) block copolymer, a styrene-butadiene-styrene (SBS) block copolymer, a styrene-isoprene-butadiene (SIB) block copolymer, or combinations thereof.

In one or more embodiments, the SBC may comprise from about 10 to about 45 wt % styrene. In the same or other embodiments, the styrenic block copolymer component may have a diblock content of from about 0 to about 85 wt %. Diblock content may be determined by GPC, and may be manipulated via the reactor settings employed to produce the styrenic block copolymer component.

In some embodiments, the SBC may comprise a styrene-isoprene-styrene (SIS) block copolymer. Such SIS block copolymers are thermoplastic elastomers having the structure (S-I)nS, where S is substantially a polystyrene block, I is substantially a polyisoprene block, and n is an integer of from about 1 to about 10. The styrene content of the SIS block copolymer is typically from about 10 to about 45 wt %, or from about 15 to about 35 wt %, or from about 20 to 30 wt %. The number average molecular weight (Mn) of the SIS block copolymer may be from about 50,000 to about 500,000. In one or more embodiments, the SIS block copolymer is a triblock copolymer of the formula above, where n=1, i.e., a linear polymer of the formula S-I-S wherein S is substantially a polystyrene block and I is substantially a polyisoprene block. These block copolymers may be prepared by well known anionic solution polymerization techniques using lithium-type initiators such as disclosed in U.S. Pat. Nos. 3,251,905 and 3,239,478, which are hereby incorporated by reference in their entireties.

The SIS block copolymers employed herein may have a number average molecular weight (determined by GPC) in the range of from about 50,000 to 500,000, or from about 70,000 to about 250,000, or from about 90,000 to about 175,000, or from about 90,000 to about 135,000. SIS block copolymers may comprise a blend of two or more different SIS copolymers, which may have the same or different styrene content, and may be blended to a ratio in the range of from 10:1 to 1:10 parts by weight. The SIS copolymer may be a pure triblock (one having less than 0.1 wt % of diblock polymer, preferably 0% diblock polymer), or may contain from about 0.1 to about 85 wt %, or from about 0.1 to about 75 wt %, or from about 1 to about 65 wt %, or from about 5 to about 50 wt % diblock copolymer having the structure S-I. This material may be present as an impurity in the manufacture of the triblock copolymer or may be separately blended with the triblock as a further technique for achieving target polystyrene content or modifying the cohesive properties of the composition. In one or more embodiments, the balance of the SIS copolymer that is not diblock copolymer may be in the radial form. In one or more embodiments, the number average molecular weight of the diblock SI copolymers may range from about 100,000 to about 250,000.

SIS copolymers may be linear or radial in structure, or a combination of the two. Radial SIS copolymers may have the same styrene levels as the linear copolymers discussed above, e.g., from about 10 to about 45 wt % polymerized styrene. Radial SIS copolymers useful in the practice of the invention may have a molecular weight (Mn) of from about 180,000 to about 250,000. Linear and radial SIS block copolymers of the type described herein are available commercially and are prepared in accordance with methods known in the art. Examples of SIS copolymers useful in the practice of this invention include those available under the trade names Vector and DPX (from Dexco Polymers LLP), Kraton (from Shell Chemical Company), Europrene (from Enichem), and Quintac (from Nippon Zeon). Particularly useful SIS block copolymers include, but are not limited to, Vector 4111, Vector 4511 and Vector 4113; DPX 552 and DPX 556; Kraton D 1107, Kraton D 1124, Kraton D 1160 and Kraton D 1161; Europrene SOL T 190 and Europrene SOL T 193; and Quintac 3421, Quintac 3422, Quintac 3433, and Quintac 3450.

In some embodiments, the SBC may comprise a styrene-butadiene-styrene (SBS) block copolymer. Such SBS block copolymers are thermoplastic elastomers having the structure (S-B)nS, where S is substantially a polystyrene block, B is substantially a polybutadiene block, and n is an integer of from about 1 to about 10. The styrene content of the SBS block copolymer is typically from about 10 to about 45 wt %, or from about 15 to about 35 wt %, or from about 20 to 30 wt %. The number average molecular weight (Mn) of the SBS block copolymer may be from about 50,000 to about 500,000. In one or more embodiments, the SBS block copolymer is a triblock copolymer of the formula above, where n=1, i.e., a linear polymer of the formula S-B-S wherein S is substantially a polystyrene block and B is substantially a polybutadiene block. These block copolymers may be prepared by well known anionic solution polymerization techniques using lithium-type initiators such as disclosed in U.S. Pat. Nos. 3,251,905 and 3,239,478, which are hereby incorporated by reference in their entireties.

SBS block copolymers employed herein may have a number average molecular weight (determined by GPC) in the range of from about 50,000 to 500,000, or from about 100,000 to about 180,000, or from about 110,000 to about 160,000, or from about 110,000 to about 140,000. The SBS copolymer may be a pure triblock (one having less than 0.1 wt % of diblock polymer, preferably 0% diblock polymer), or may contain from about 0.1 to about 85 wt %, or from about 0.1 to about 75 wt %, or from about 1 to about 65 wt %, or from about 5 to about 50 wt % diblock copolymer having the structure S-B. This material may be present as an impurity in the manufacture of the triblock copolymer or may be separately blended with the triblock as a further technique for achieving target polystyrene content or modifying the cohesive properties of the composition. In one or more embodiments, the balance of the SBS copolymer that is not diblock copolymer may be in the radial form. In one or more embodiments, the number average molecular weight of the diblock SB copolymers may range from about 100,000 to about 250,000.

SBS copolymers may be linear or radial in structure, or a combination of the two. Radial SBS copolymers may have the same styrene levels as the linear copolymers discussed above, e.g., from about 10 to about 45 wt % polymerized styrene. Linear and radial SBS block copolymers of the type described herein are available commercially and are prepared in accordance with methods known in the art. Examples of SBS copolymers useful in the practice of this invention include those available under the tradenames Vector (from Dexco Polymers LLP), Kraton (from Shell Chemical Company), Europrene (from Enichem), and Finaprene (from Fina Chemicals). Particularly useful SBS block copolymers include, but are not limited to, Vector 8505, Kraton D 1102, Kraton D 4141, Kraton D 4158, Europrene SOL T 166, and Finaprene 411.

Radial styrenic block copolymers and other styrenic block copolymers suitable for use in the present invention include those described in U.S. Application Publication No. 2009/0133834, which is incorporated by reference herein in its entirety.

Sheath Polymer

Polymers intended for use as the sheath polymer in the fibers described herein include any polymers suitable for use as feel modifiers in elastic nonwoven fabrics and articles that have a low molecular weight relative to the core polymer and limited miscibility in the core polymer. In one embodiment, such polymers include a polypropylene homopolymer such as Achieve™ 6936G1 from ExxonMobil Chemical Co. Such polymers include, but are not limited to, olefin waxes including propylene waxes and ethylene waxes and combinations thereof. In certain embodiments of the invention, the sheath polymer has a low molecular weight relative to the molecular weight of the sheath polymer. For example, in some embodiments, the sheath polymer has a weight average molecular weight (M_(w)) that is at least about 20,000 g/mol less, or at least about 50,000 g/mol less, or at least about 75,000 g/mol less, or at least about 100,000 g/mol less than the M_(w) of the core polymer. In the same or other embodiments, the sheath polymer has an M_(w) less than about 65,000 g/mol, or less than about 50,000 g/mol, or less than about 45,000 g/mol, or less than about 40,000 g/mol, or less than about 35,000 g/mol or less than about 30,000 g/mol, or less than about 25,000 g/mol, or less than about 20,000 g/mol, or less than about 15,000 g/mol, or less than about 10,000 g/mol.

Olefin waxes suitable for use as the sheath polymer may be polar or nonpolar, branched or unbranched, and may be prepared using any suitable catalyst system including Ziegler-Natta catalysts, Phillips-type catalysts, chromium based catalysts, and metallocene catalyst systems. The olefin waxes may be low, medium, or high density, such that in some embodiments of the invention, the waxes may have a density ranging from about 0.88 g/cm³ to about 1.0 g/cm³, or from about 0.89 g/cm³ to about 0.99 g/cm³, or from about 0.90 g/cm³ to about 0.98 g/cm³. In the same or other embodiments, the waxes may have a viscosity from about 100 to about 2000 mPa·s, or from about 200 to about 1900 mPas, or from about 300 to about 1800 mPa·s.

In one or more embodiments, the sheath polymer can be grafted or functionalized using one or more grafting monomers. The grafting monomer can be or include at least one ethylenically unsaturated carboxylic acid or acid derivative, such as an acid anhydride, ester, salt, amide, imide, acrylates or the like. Illustrative monomers include but are not limited to acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, maleic anhydride, 4-methyl cyclohexene-1,2-dicarboxylic acid anhydride, bicyclo(2.2.2)octene-2,3-dicarboxylic acid anhydride, 1,2,3,4,5,8,9,10-octahydronaphthalene-2,3-dicarboxylic acid anhydride, 2-oxa-1,3-diketospiro(4.4)nonene, bicyclo(2.2.1)heptene-2,3-dicarboxylic acid anhydride, maleopimaric acid, tetrahydrophthalic anhydride, norbornene-2,3-dicarboxylic acid anhydride, nadic anhydride, methyl nadic anhydride, himic anhydride, methyl himic anhydride, and 5-methylbicyclo(2.2.1)heptene-2,3-dicarboxylic acid anhydride. Other suitable grafting monomers include methyl acrylate and higher alkyl acrylates, methyl methacrylate and higher alkyl methacrylates, acrylic acid, methacrylic acid, hydroxy-methyl methacrylate, hydroxyl-ethyl methacrylate and higher hydroxy-alkyl methacrylates and glycidyl methacrylate. Maleic anhydride is a preferred grafting monomer. In certain embodiments herein, the sheath polymer may be a grafted polymer having a polyethylene or polypropylene backbone with maleic anhydride grafted to the backbone. In certain other embodiments, the sheath polymer may be a polyethylene or polypropylene wax having at least one functionalized end group, such as for example vinyl tetramethylene (VTM), providing the polymer with a polar character.

Exemplary olefin waxes suitable for use as the sheath polymer in the present invention include, but are not limited to, those commercially available under the names Licowax and Licocene (particularly Licowax PE 130, Licowax PE 520, Licocene PE 5301, and Licocene PP 7502) from Clariant Chemicals, Honeywell A-C performance additives (particularly A-C 9) from Honeywell International, and Achieve™ (particularly Achieve 6936G1) from ExxonMobil Chemical Co.

Preparation of Bicomponent Fibers and Fabrics

The present invention is directed not only to bicomponent fibers, but also to methods for preparing those fibers. To form the bicomponent fibers of the invention, a molten blend is prepared comprising the core polymer and the sheath polymer. The molten blend is then extruded using an extruder having a die with a length to diameter ratio greater than or equal to about 10 and under shear conditions sufficient to drive the sheath polymer toward the die wall. The resulting bicomponent fibers have a core comprising the core polymer and a sheath comprising the sheath polymer. While the terms “core” and “sheath” are used herein for ease of reference, it should be recognized that the polymers may not completely separate during the extrusion process, and there may not be a defined boundary between the core and sheath of the bicomponent fibers described herein. Rather, fibers of the invention also include those fibers having a cross-sectional gradient, in which the concentration of the sheath polymer is highest at the surface of the fiber, and the concentration of the core polymer is highest at the center of the fiber.

Molten blends comprising the core polymer and the sheath polymer may be prepared by any method that provides an intimate mixture of the components. Blending and homogenation of polymers are well known in the art and include single and twin screw mixing extruders, static mixers for mixing molten polymer streams of low viscosity, impingement mixers, as well as, other machines and processes designed to disperse the first and second polymers in intimate contact. For example, the polymer components and other minor components can be blended by melt blending or dry blending in continuous or batch processes. These processes are well known in the art and include single and twin screw compounding extruders, as well as, other machines and processes designed to melt and homogenize the polymer components intimately. The melt blending or compounding extruders usually are equipped with a pelletizing die to convert the homogenized polymer into pellet form. The homogenized pellets can then be fed to the extruder of fiber or nonwoven process equipment to produce fiber or fabrics. Alternately, the first and second polymers may be dry blended and fed to the extruder of the nonwoven process equipment. The blend of the core and sheath polymers may also be produced by any reactor blend method currently known in the art. A reactor blend is a highly dispersed and mechanically inseparable blend of the polymers produced in situ as the result of sequential polymerization of one or more monomers with the formation of one polymer in the presence of another. The polymers may be produced in any of the polymerization methods described above. The reactor blends may be produced in a single reactor or in two or more reactors arranged in series. The blend of the core and sheath polymers may further be produced by combining reactor blending with post reactor blending.

The blended polymer resin may be used to produce fibers and nonwoven fabric products. As used herein, “nonwoven” refers to a textile material that has been produced by methods other than weaving. In nonwoven fabrics, the fibers are processed directly into a planar sheet-like fabric structure, and are then bonded chemically or thermally, or interlocked mechanically (or some combination thereof) to achieve a cohesive fabric.

The nonwoven fibers and fabrics of the present invention can be formed by any method known in the art. Preferably, the nonwoven fibers are produced by a meltblown or spunbond process.

In a typical spunbond process, polymer is supplied to a heated extruder to melt and homogenize the polymers. The extruder supplies melted polymer to a spinneret where the polymer is fiberized as passed through fine openings arranged in one or more rows in the spinneret, forming a curtain of filaments. The filaments are usually quenched with air at a low temperature, drawn, usually pneumatically, and deposited on a moving mat, belt or “forming wire” to form the nonwoven fabric. See, for example, in U.S. Pat. Nos. 4,340,563; 3,692,618; 3,802,817; 3,338,992; 3,341,394; 3,502,763; and 3,542,615.

The fibers produced in the spunbond process are usually in the range of from about 10 to about 50 microns in diameter, depending on process conditions and the desired end use for the fabrics to be produced from such fibers. For example, increasing the polymer molecular weight or decreasing the processing temperature results in larger diameter fibers. Changes in the quench air temperature and pneumatic draw pressure also have an affect on fiber diameter.

In a typical meltblown process, an extruder delivers molten polymer to a metering melt pump. The melt pump delivers the molten polymer at a steady output rate to a special melt blowing die, which may comprise a plurality of fine, usually circular, die capillaries (also called spinnerets). As the molten polymer filaments exit the die, they are contacted by high temperature, high velocity air (called process or primary air). This air rapidly draws and, in combination with the quench air, solidifies the filaments. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web or nonwoven fabric of randomly dispersed meltblown fibers. Such a process is generally described in, for example, U.S. Pat. Nos. 3,849,241 and 6,268,203. Meltblown fibers are microfibers that are either continuous or discontinuous, and, depending on the resin, may be smaller than about 10 microns (for example, for high MFR isotactic polypropylene resins such as PP3746G or Achieve™ 6936G1, available from ExxonMobil Chemical Company); whereas for certain resins (for example, Vistamaxx™ propylene-based elastomer, available from ExxonMobil Chemical Company) or certain high throughput processes such as those described herein, meltblown fibers may have diameters greater than 10 microns, such as from about 10 to about 30 microns, or about 10 to about 15 microns. The term meltblowing as used herein is meant to encompass the meltspray process.

It is known that polymer flow under conditions of high shear can induce segregation of molecules. The extent of segregation and time required for segregation depend upon the amount of shear produced. In turn, the level of shear depends upon the process conditions and the level of immiscibility of the polymer components. Immiscibility of polymer blend components is a result of compositional differences and differences in molecular weight of the blend components, with greater differences leading to a higher probability of segregation. Process conditions that create shear and therefore influence segregation include flow velocity (which is related to the pressure through the die), and the size and/or cross-sectional area of the die. Segregation is also affected by the temperature and length of time that the polymer blend is exposed to the high shear environment. The residence time is particularly important, because it must be long enough to ensure that the molecules of the low molecular weight sheath polymer have sufficient time to migrate to the polymer/die wall interface. Capillaries with longer length to diameter (L/D) ratios typically result in longer residence times, thus leading to better segregation of the polymer components. Conventional meltblown dies typically have an L/D ratio less than 10, usually less than 5. To form the bicomponent fibers described herein, the blend of core and sheath polymers is meltblown through a die with a larger L/D ratio, for example at least 10, or greater than 10, greater than 15, greater than 20, greater than 25, greater than 30, greater than 50, greater than 75, or greater than 100. Under the proper processing conditions such as those described herein, sufficient shear is developed using a meltblown die with a high L/D ratio to drive the sheath polymer to the die wall. The resulting bicomponent fibers have a sheath comprising the sheath polymer and a core comprising the core polymer. Such fibers have properties ideal for use in nonwoven fabric applications, because they have a more desirable feel than previous fibers due to the sheath layer, while continuing to provide excellent elastic properties due to the core layer.

Fabrics formed from the bicomponent fibers described herein may be a single layer, or may be multilayer laminates. One application is to make a laminate (or “composite”) from meltblown fabric (“M”) and spunbond fabric (“S”), which combines the advantages of strength from spunbonded fabric and greater barrier properties of the meltblown fabric. A typical laminate or composite has three or more layers, a meltblown layer(s) sandwiched between two or more spunbonded layers, or “SMS” fabric composites. Examples of other combinations are SSMMSS, SMMS, and SMMSS composites. Composites can also be made of the meltblown fabrics of the invention with other materials, either synthetic or natural, to produce useful articles.

In some embodiments of the invention, the fibers or fabrics described herein may be annealed. Annealing partially relieves the internal stress in a stretched fiber and restores the elastic recovery properties of the core polymer in the fiber. Annealing has been shown to lead to significant changes in the internal organization of the crystalline structure and the relative ordering of the semi-amorphous and semi-crystalline phases. This leads to recovery of the elastic properties. For example, annealing the fiber at a temperature of at least 40° C., above room temperature but slightly below the crystalline melting point of the blend, is adequate for the restoration of the elastic properties in the fiber. Thermal annealing of a polymer blend is conducted by maintaining the polymer blends or the articles made from a such a blend at a temperature, for example, between about 25° C. and about 160° C., or alternatively between about 60° C. and about 130° C., for a period between a few seconds and about 1 hour. A typical annealing period is 1 to 5 minutes at 100° C. In nonwoven processes, the fabric web usually passes through a calender to point bond (consolidate) the web. The passage of the unconsolidated nonwoven web through a heated calender at relatively high temperature is sufficient to anneal the fiber and increase the elasticity of the nonwoven web. The nonwoven web should be under low tension to allow for shrinkage of the web in both machine direction (MD) and transverse direction (TD) to enhance the elasticity of the nonwoven web. In some embodiments, the bonding calender roll temperature may range from about 60° C. to about 130° C. In another embodiment, the temperature is about 100° C. The annealing time and temperature may be adjusted depending upon the composition of a particular polymer blend.

A variety of additives may be incorporated into the polymers used to make the fibers and fabrics described herein, depending upon the intended purpose. Such additives may include, but are not limited to, stabilizers, antioxidants, fillers, colorants, nucleating agents, dispersing agents, mold release agents, slip agents, fire retardants, plasticizers, pigments, vulcanizing or curative agents, vulcanizing or curative accelerators, cure retarders, processing aids, tackifying resins, and the like. Other additives may include fillers and/or reinforcing materials, such as carbon black, clay, talc, calcium carbonate, mica, silica, silicate, combinations thereof, and the like. Primary and secondary antioxidants include, for example, hindered phenols, hindered amines, and phosphates. Nucleating agents include, for example, sodium benzoate and talc. Also, to improve crystallization rates, other nucleating agents may also be employed such as Ziegler-Natta olefin products or other highly crystalline polymers. Other additives such as dispersing agents, for example, Acrowax C, can also be included. Slip agents include, for example, oleamide and erucamide. Catalyst deactivators are also commonly used, for example, calcium stearate, hydrotalcite, and calcium oxide, and/or other acid neutralizers known in the art.

The nonwoven products described above may be used in many articles such as hygiene products including, but not limited to, diapers, feminine care products, and adult incontinent products. The nonwoven products may also be used in medical products such as sterile wrap, isolation gowns, operating room gowns, surgical gowns, surgical drapes, first aid dressings, and other disposable items.

Examples

The following designations are used herein to refer to the core (C) and sheath (S) polymers used in the following examples.

Polymer C1 is a propylene-based polymer comprising propylene and ethylene, having an ethylene content of about 15 wt %, an MFR of about 18 g/10 min (230° C., 2.16 kg), and an Mw of about 130,000 g/mol.

Polymer C2 is an ethylene-propylene copolymer rubber, having an ethylene content of about 72 wt %, a melt index of about 1.0 g/10 min (190° C., 2.16 kg), and an Mw of about 250,000 g/mol.

Polymer C3 is an SEBS linear triblock block copolymer having a styrene content of about 20 wt % and an Mw of about 130,000 g/mol.

Polymer C4 is an ethylene-based olefin block copolymer having a melt index of about 5.0 g/10 min (190° C., 2.16 kg) and an Mw of about 130,000 g/mol.

Polymer C5 is a propylene-based polymer comprising propylene and ethylene, having an ethylene content of about 13 wt %, an MFR of about 79 g/10 min (230° C., 2.16 kg), and an Mw of about 142,000 g/mol.

Polymer S1 is a polyethylene wax having a viscosity of about 300 mPa·s, a density of about 0.96-0.98 g/cm³, and an Mw of about 14,850 g/mol.

Polymer S2 is a polyethylene wax having a viscosity of about 650 mPa·s, a density of about 0.92-0.94 g/cm³, and an Mw of about 6,800 g/mol.

Polymer S3 is a polyethylene homopolymer wax having a viscosity of about 450 mPa·s and an Mw of about 7,000 g/mol.

Polymer S4 is a metallocene-catalyzed polyethylene wax having a viscosity of about 350 mPa·s, a density of about 0.96-0.98 g/cm³, and an Mw of about 4,300 g/mol.

Polymer S5 is a polypropylene wax having a viscosity of about 1800 mPa·s, a density of about 0.90 g/cm³, and an Mw of about 27,300 g/mol.

Polymer S6 is a high melt flow rate homopolypropylene having an MFR of about 1550 g/10 min (230° C., 2.16 kg) and an Mw of about 63,000 g/mol.

Various polymer blends were formed as set forth in Table 1. Example No. 1 2 3 4 5 6 7 8 9 10 11 Core Polymer, wt % C1 85 85 85 85 50 70 80 C2 85 C3 85 C4 85 C5 90 Sheath Polymer, wt % S1 15 S2 15 S3 15 S4 15 S5 15 15 15 S6 10 50 30 20 Blend MFR, g/10 min 35.2 33.2 30.4 32.3 11.2 40.5 (230° C., 2.16 kg)

Having described the various aspects of the compositions herein, further specific embodiments of the invention include those set forth in the following lettered paragraphs:

-   -   A. A bicomponent polymer fiber having a core comprising a core         polymer and a sheath comprising a sheath polymer, wherein the         sheath polymer is a polyolefin having a weight average molecular         weight (Mw) less than about 65,000 g/mol and wherein the Mw of         the core polymer is at least about 20,000 g/mol greater than the         Mw of the sheath polymer.     -   B. The polymer fiber of paragraph A, wherein the core polymer is         selected from a propylene-based polymer, an ethylene-based         polymer, a propylene-ethylene block copolymer, a styrenic block         copolymer, an acrylate, or a combination of the foregoing.     -   C. The polymer fiber of any of paragraphs A-B, wherein the core         polymer is a propylene-based polymer comprising from about 5 to         about 30 wt % ethylene and/or a C₄-C₁₂ alpha olefin and having a         triad tacticity greater than about 90% and a heat of fusion less         than about 75 J/g.     -   D. The polymer fiber of any of paragraphs B-C, wherein the         propylene-based polymer comprises from about 8 to about 20 wt %         ethylene.     -   E. The polymer fiber of any of paragraphs B-D, wherein the         propylene-based polymer comprises from about 12 to about 18 wt %         ethylene.     -   F. The polymer fiber of any of paragraphs A-E, wherein the Mw of         the core polymer is at least about 50,000 g/mol, or at least         about 75,000 g/mol greater than the Mw of the sheath polymer.     -   G. The polymer fiber of any of paragraphs A-F, wherein the Mw of         the core polymer is at least about 100,000 g/mol greater than         the Mw of the sheath polymer.     -   H. The polymer fiber of any of paragraphs A-G, wherein the Mw of         the sheath polymer is less than about 50,000 g/mol and the Mw of         the core polymer is greater than about 100,000 g/mol, or wherein         the Mw of the sheath polymer is less than about 30,000 g/mol and         the Mw of the core polymer is greater than about 125,000 g/mol.     -   I. The polymer fiber of any of paragraphs A-H, wherein the         sheath polymer is selected from a polypropylene wax, a         polyethylene wax, and combinations thereof.     -   J. The polymer fiber of any of paragraphs A-I, wherein the         sheath polymer comprises a backbone of propylene or ethylene         with maleic anhydride grafted to the backbone.     -   K. The polymer fiber of any of paragraphs A-J, wherein the         sheath polymer is a polyethylene or polypropylene wax comprising         at least one functionalized end group providing a polar         character to the polymer.     -   L. A nonwoven fabric comprising polymer fibers according to any         of paragraphs A-K.     -   M. A process for forming bicomponent polymer fibers comprising:         -   i. forming a molten blend of a core polymer and a sheath             polymer;         -   ii. extruding the molten polymer blend using an extruder             having a die with a length to diameter ratio greater than or             equal to about 10 and under shear conditions sufficient to             drive the sheath polymer to the die wall; and         -   iii. forming meltblown fibers having a core comprising the             core polymer and a sheath comprising the sheath polymer;     -   wherein the sheath polymer is a polyolefin having a weight         average molecular weight (Mw) less than about 65,000 g/mol and         wherein the Mw of the core polymer is at least about 20,000         g/mol greater than the Mw of the sheath polymer.     -   N. The process of paragraph M, wherein the core polymer is         selected from a propylene-based polymer, an ethylene-based         polymer, a propylene-ethylene block copolymer, a styrenic block         copolymer, an acrylate, or a combination of the foregoing.     -   O. The process of any of paragraphs M-N, wherein the core         polymer is a propylene-based polymer comprising from about 5 to         about 30 wt % ethylene and/or a C₄-C₁₂ alpha olefin and having a         triad tacticity greater than about 90% and a heat of fusion less         than about 75 J/g.     -   P. The process of any of paragraphs N-O, wherein the propylene         based polymer comprises from about 8 to about 20 wt % ethylene.     -   Q. The process of any of paragraphs N-P, wherein the propylene         based polymer comprises from about 12 to about 18 wt % ethylene.     -   R. The process of any of paragraphs M-Q, wherein the Mw of the         core polymer is at least about 50,000 g/mol, or at least about         75,000 g/mol greater than the Mw of the sheath polymer.     -   S. The process of any of paragraphs M-R, wherein the Mw of the         core polymer is at least about 100,000 g/mol greater than the Mw         of the sheath polymer.     -   T. The process of any of paragraphs M-S, wherein the Mw of the         sheath polymer is less than about 50,000 g/mol and the Mw of the         core polymer is greater than about 100,000 g/mol, or wherein the         Mw of the sheath polymer is less than about 30,000 g/mol and the         Mw of the core polymer is greater than about 125,000 g/mol.     -   U. The process of any of paragraphs M-T, wherein the sheath         polymer is selected from a polypropylene wax, a polyethylene         wax, and combinations thereof     -   V. The process of any of paragraphs M-U, wherein the sheath         polymer comprises a backbone of propylene or ethylene with         maleic anhydride grafted to the backbone.     -   W. The process of any of paragraphs M-V, wherein the sheath         polymer is a polyethylene or polypropylene wax comprising at         least one functionalized end group providing a polar character         to the polymer.     -   X. A nonwoven fabric comprising polymer fibers made according to         any of paragraphs M-W.     -   Y. An article comprising a nonwoven fabric according to any of         paragraphs M-X.     -   Z. A fiber according to any of paragraphs A-Y, further         comprising at least one of carbon black, clay, talc, calcium         carbonate, mica, silica, and silicate.     -   AA. A fiber according to any of paragraphs A-Z, further         comprising at least one of carbon black, clay, talc, calcium         carbonate, mica, silica, silicate, hindered phenol, hindered         amine, phosphate, sodium benzoate, oleamide, erucamide, calcium         stearate, hydrotalcite, and calcium oxide.

Certain embodiments and features have been described using a set of numerical upper limits and a set of numerical lower limits. It should be appreciated that ranges from any lower limit to any upper limit are contemplated unless otherwise indicated. Certain lower limits, upper limits and ranges appear in one or more claims below. All numerical values are “about” or “approximately” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

To the extent a term used in a claim is not defined above, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Furthermore, all patents, test procedures, and other documents cited in this application are fully incorporated by reference to the extent such disclosure is not inconsistent with this application and for all jurisdictions in which such incorporation is permitted.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. 

1. A bicomponent polymer fiber having a core comprising a core polymer and a sheath comprising a sheath polymer, wherein the sheath polymer is a polyolefin having a weight average molecular weight (Mw) less than about 65,000 g/mole and wherein the Mw of the core polymer is at least about 20,000 g/mole greater than the Mw of the sheath polymer.
 2. The polymer fiber of claim 1, wherein the core polymer is selected from a propylene-based polymer, an ethylene-based polymer, a propylene-ethylene block copolymer, a styrenic block copolymer, an acrylate, or a combination of the foregoing.
 3. The polymer fiber of claim 1, wherein the core polymer is a propylene-based polymer comprising from about 5 to about 30 wt % ethylene and/or a C₄-C₁₂ alpha olefin and having a triad tacticity greater than about 90% and a heat of fusion less than about 75 J/g.
 4. The polymer fiber of claim 1, wherein the propylene-based polymer comprises from about 8 to about 20 wt % ethylene.
 5. The polymer fiber of claim 1, wherein the propylene-based polymer comprises from about 12 to about 18 wt % ethylene.
 6. The polymer fiber of claim 1, wherein the Mw of the core polymer is at least about 75,000 g/mole greater than the Mw of the sheath polymer.
 7. The polymer fiber of claim 1, wherein the Mw of the core polymer is at least about 100,000 g/mole greater than the Mw of the sheath polymer.
 8. The polymer fiber of claim 1, wherein the Mw of the sheath polymer is less than about 50,000 g/mole and the Mw of the core polymer is greater than about 100,000 g/mole.
 9. The polymer fiber of claim 1, wherein the sheath polymer is selected from a polypropylene wax, a polyethylene wax, and combinations thereof.
 10. The polymer fiber of claim 1, wherein the sheath polymer comprises a backbone of propylene or ethylene with maleic anhydride grafted to the backbone.
 11. The polymer fiber of claim 1, wherein the sheath polymer is a polyethylene or polypropylene wax comprising at least one functionalized end group providing a polar character to the polymer.
 12. A nonwoven fabric comprising polymer fibers according to claim
 1. 13. A process for forming bicomponent polymer fibers comprising: a. forming a molten blend of a core polymer and a sheath polymer; b. extruding the molten polymer blend using an extruder having a die with a length to diameter ratio greater than or equal to about 10 and under shear conditions sufficient to drive the sheath polymer to the die wall; and c. forming meltblown fibers having a core comprising the core polymer and a sheath comprising the sheath polymer; wherein the sheath polymer is a polyolefin having a weight average molecular weight (Mw) less than about 65,000 g/mole and wherein the Mw of the core polymer is at least about 20,000 g/mole greater than the Mw of the sheath polymer.
 14. The process of claim 13, wherein the core polymer is selected from a propylene-based polymer, an ethylene-based polymer, a propylene-ethylene block copolymer, a styrenic block copolymer, an acrylate, or a combination of the foregoing.
 15. The process of claim 13, wherein the core polymer is a propylene-based polymer comprising from about 5 to about 30 wt % ethylene and/or a C₄-C₁₂ alpha olefin and having a triad tacticity greater than about 90% and a heat of fusion less than about 75 J/g.
 16. The process of claim 13, wherein the propylene based polymer comprises from about 8 to about 20 wt % ethylene.
 17. The process of claim 13, wherein the Mw of the core polymer is at least about 75,000 g/mole greater than the Mw of the sheath polymer.
 18. The process of claim 13, wherein the Mw of the core polymer is at least about 100,000 g/mole greater than the Mw of the sheath polymer.
 19. The process of claim 13, wherein the Mw of the sheath polymer is less than about 50,000 g/mole and the Mw of the core polymer is greater than about 100,000 g/mole.
 20. The process of claim 13, wherein the sheath polymer is selected from a polypropylene wax, a polyethylene wax, and combinations thereof.
 21. The process of claim 13, wherein the sheath polymer comprises a backbone of propylene or ethylene with maleic anhydride grafted to the backbone.
 22. The process of claim 13, wherein the sheath polymer is a polyethylene or polypropylene wax comprising at least one functionalized end group providing a polar character to the polymer.
 23. A nonwoven fabric comprising polymer fibers made according to claim
 13. 24. An article comprising a nonwoven fabric according to claim
 23. 25. The fiber according claim 1, further comprising at least one of carbon black, clay, talc, calcium carbonate, mica, silica, silicate, hindered phenol, hindered amine, phosphate, sodium benzoate, oleamide, erucamide, calcium stearate, hydrotalcite, and calcium oxide. 